Overall, Smilodon was more robustly built than any modern cat, with particularly well-developed forelimbs and exceptionally long upper canines. Its jaw had a bigger gape than modern cats and its upper canines were slender and fragile, being adapted for precision killing. These attributes made Smilodon a specialized hunter of large herbivores like bison and camels.

Smilodon likely lived in closed habitats like forests and bush which would have provided cover for ambushing prey. Its reliance on large animals may have been the cause of its extinction. Scientists debate over whether Smilodon was a social animal. Comparison of predator responses to distress calls and the prevalence of healed injuries suggest it was social, while its small brain size and vegetated habitat suggest it was more solitary. Some fossils show signs of ankylosing spondylitis, trauma and arthritis. Smilodon went extinct 10,000 years ago.

A number of Smilodon species have been described, but today usually only three are recognized.[9]

Smilodon gracilis, 2.5 million–500,000 years ago; the smallest and earliest species, estimated at 55 to 100 kg (120 to 220 lb) in weight[10] was the successor of Megantereon in North America, from which it probably evolved. This species reached the north of South America in the early Pleistocene, along with Homotherium.[11] The other Smilodon species probably derived from this species. As its specific name suggests, this species is the most lightly built of the genus.[3]

Smilodon populator, 1 million–10,000 years ago; occurred in the eastern parts of South America and was larger than the North American species.[15] It is perhaps the largest known felid, with a body mass range of 220 to 400 kg (490 to 880 lb).[10][16] It stood at a shoulder height of 120 cm (47 in).[9] Compared to S. fatalis, S. populator had a more elongated and narrow skull, higher positioned nasals, more massive metapodials and slightly longer forelimbs relative to hindlimbs.[12]

Smilodon was around the size of a modern lion or tiger but was more robustly built. It had a reduced lumbar region, high scapula, short tail, broad limbs with relatively short feet and long canines.[9][17] The brain of Smilodon was relatively small compared to other cat species.[18] In reconstructing the facial appearance of Smilodon, Miller (1969) proposed that it looked very different from a typical cat: having a lower lip line (to allow its mouth to open so wide without tearing the facial tissues), a more retracted nose and lower placed ears.[19] However this is disputed, and Antón, et al. (1998) write that the facial features of Smilodon were overall no different than that of other cats.[20]

There is some dispute over whether Smilodon was sexually dimorphic. Some studies of Smilodon fatalis fossils have found little difference between the sexes.[21][22] Conversely a 2012 study found that while fossils of S. fatalis show less variation in size among individuals than modern Panthera, they do appear to show the same difference in some traits between the sexes.[23]

Smilodon had shorter and more massive limbs than other felids. It had well developed flexors and extensors in its forearms, which enabled it to pull down and securely hold down large prey so it could deliver a killing bite without endangering the vulnerable elongate canines. Analysis of the cross-sections of S. fatalishumeri indicated that they were strengthened by cortical thickening to such an extent that they would have been able to sustain greater loading than those of extant big cats, or of the extinct American lion. However, the thickening of S. fatalisfemurs was within the range of extant felids.[24] The heel bone of Smilodon was fairly long which suggests it was a good jumper.[9]

Smilodon is most famous for its relatively long canines, which are the longest found in the saber-toothed cats, at about 28 cm (11 in) long in the largest species Smilodon populator.[9] Those of S. fatalis reached their full size in 18 months at a growth rate of 7 mm/month.[25] These canine teeth were slender and had fine serrations.[8] They were fragile and could not have bitten into bone; thus, these cats did not use their long teeth while taking down prey, due to the risk of breaking. Only when their prey was totally subdued did they use their teeth to slash the throat.[24]

Despite being more powerfully built than other large cats, Smilodon actually had a weaker bite. Modern big cats have more pronounced zygomatic arches, while Smilodon had smaller zygomatic arches which restricted the thickness and therefore power of the temporalis muscles, and thus reduced Smilodon's bite force. Analysis of its narrow jaws indicates it could produce a bite only a third as strong as that of a lion.[26] There seems to a be a general rule that the saber-toothed cats with the largest canines had proportionally weaker bites. However, analyses of canine bending strength (the ability of the canine teeth to resist bending forces without breaking) and bite forces indicate that the saber-toothed cats' teeth were stronger relative to the bite force than those of modern "big cats".[27] In addition, Smilodon's gape could have reached almost 120 degrees,[28] while that of the modern lion reaches 65 degrees.[29] This makes the gape wide enough to allow Smilodon to grasp large prey despite the long canines.[20]

Smilodon was likely an ambush predator and concealed itself in dense vegetation.[35] It probably used its great upper-body strength to wrestle prey to the ground, where its long canines could deliver a deep stabbing bite to the throat which would generally cut through the jugular vein and/or the trachea and thus kill the prey very quickly.[24][36] By contrast, modern cats kill large prey with a suffocating bite.[24] An alternate hypothesis suggests that Smilodon targeted the belly of its prey. However, this is disputed as the curvature of their prey's belly would likely have prevented the cat from getting a good bite.[37]Smilodon probably avoided eating bone and would have left enough food for scavengers.[38]Smilodon itself may have scavenged dire wolf kills.[39]

Scientists debate whether Smilodon was social. One study of African predators found that social predators like lions, spotted hyenas respond more to the distress calls of prey than solitary species. Since Smilodon fatalis fossils are common at the La Brea Tar Pits and were likely attracted by the distress calls of stuck prey, this could mean that La Brea S. fatalis were social as well.[40] Critics claim the study neglects other factors, such as body mass (heavier animals are more likely to get stuck then lighter ones), intelligence (some social animals like the American lion may have avoided the tar because they were better able to recognize the hazard), lack of visual and olfactory lures, the type of audio lure, and the length of the distress calls (the actual distress calls of the trapped prey animals would have lasted longer than the calls used in the study). In addition, they note that solitary cats like tigers are known to aggregate around a single carcass.[41] The authors of the original study have responded to these criticisms.[42]

Another argument for sociality is based on the healed injuries in several Smilodon fossils which would suggest that the animals needed others to provide it food.[43] This argument has been questioned, as cats can recover quickly from even severe bone damage and an injured Smilodon could survive as long as it had access to water.[44] Some researchers have argued that Smilodon's brain would have been too small for it to have been a social animal.[18] However, an analysis of brain size in living big cats found no correlation between brain size and sociality.[45] Another argument against Smilodon being social is based on it being an ambush hunter in closed habitat which would likely have negated sociality.[44]

Whether Smilodon was sexually dimorphic has implications for its reproductive behavior. Based on their conclusions that Smilodon fatalis had no sexual dimorphism, Van Valenburgh and Sacco (2002) suggest that if the cats were social they would likely have lived in monogamous pairs (along with offspring) with no intense competition among males for females.[21] Likewise, Meachen-Samuels and Binder (2010) conclude that aggression between males was less pronounced in S. fatalis than the American lion.[22] However, Christiansen and Harris (2012) find that as S. fatalis did exhibit some sexual dimorphism, there would have been evolutionary selection for competition between males.[23]

In recent years, Smilodon cubs have been found in tar pits close to the Rancho La Brea site, near the bones of larger adults. Unlike the mature specimens, juvenile Smilodon had smaller saber teeth which were used more for cutting flesh away from bones than killing prey. This likely implies that they were coming in to feed after the adults had made a kill and fed on parts of the kill that were inaccessible to the adults. In attempting to feed, the cubs and subadults would also become mired and starve to death. This behavior was recently revealed in popular culture in the first episode of the 2013 BBC documentary miniseries "Ice Age Giants".[46]

Several Smilodon fossils show signs of ankylosing spondylitis, hyperostosis and trauma;[47] some also had arthritis which gave them fused vertebrae. One study of 1,000 Smilodon skulls found that 30% of them had eroded parietal bones, which is where the largest jaw muscles attach. They also showed signs of microfractures, and the weakening and thinning of bones possibly caused by mechanical stress from the constant need to make stabbing motions with the canines.[48]

Smilodon went extinct 10,000 years ago, along with most of the Pleistocene megafauna, in the Quaternary extinction event. Its extinction may be linked to the decline and extinction of large herbivores, which were replaced by smaller and more agile ones like deer. Hence Smilodon could have been too specialized at hunting large prey and may have been unable to adapt.[24] However, a 2012 study of Smilodon tooth wear found no evidence that they were limited by food resources. Other explanations include climate change and competition with humans.[49]